The invention relates generally to the problem of limited service life in multi-cell batteries, and relates more specifically to approaches for balancing of charge among cells.
Many users want appliances to be portable, even if the appliances have historically been fixed in position due to the need for power and other connections. For many appliances, it is a straightforward matter to make the appliance portable by shrinkng it (making it smaller) and by powering it with battery power rather than line power. For those who are given the task of making an appliance portable, however, it soon becomes clear that there are many problems to be solved before the appliance will fill user expectations. For example, in a personal computer or personal digital assistant or wireless telephone, the user naturally wants long battery life as well as minimal weight. It is further desirable that battery life be long in two distinct sensesxe2x80x94the battery should last a long time in use, between rechargings; and the battery should survive a large number of charge/discharge cycles. Where disposable batteries are used, the former is the only concern, but many users prefer to use rechargeable batteries for which both senses of the term xe2x80x9clong lifexe2x80x9d are relevant.
One way to try to maximize battery life is by selecting a battery technology appropriately for the task. Some early personal computers used sealed lead-acid batteries. For many years the prevailing battery technology was nickel-cadmium More recent choices have included nickel-metal-hydride and lithium ion technology. From the user""s point of view, it is desirable to select a technology that offers a high specific capacity (capacity versus weight) so as to reduce the weight of the appliance.
These xe2x80x9clong lifexe2x80x9d goals are not easy to meet. In any battery (defined as a plurality of cells in series) there is the problem that the cells are not perfectly identical, even though the battery manufacturer will try as hard as possible to match the cells with each other. Because the cells are not identical, they do not have identical capacities. If a battery is allowed to run down nearly to full discharge, one cell will reach full discharge before its neighbors, and will suffer chemical degradation if current continues to be forced through it (by its neighbors) during further discharge. This so-called xe2x80x9creverse chargingxe2x80x9d problem can ruin a cell in a very short time, depending on its particular technology.
Yet another problem arises when the battery is to be recharged. Suppose the cells were to have identical charging qualities, that is, that a particular charge forced through each cell would give rise to precisely the same extent of recharging. Even with such cells, the problem is that they might not have begun with the same level of charge (due to previous use, for example). As such, if they are series-connected and given a particular amount of charging current, they would not reach full charge simultaneously. As a general matter, one cell would reach full charge sooner than its neighbors. Such a cell faces the problem of having to find some way to dissipate excess energy, typically through radiated heat, during the time when the remaining cells continue to be charged. This xe2x80x9coverchargingxe2x80x9d problem can damage a cell through any of a number of mechanisms (depending on the cell technology) including the conversion of water to steam.
In real life, cells do not have identical charging qualities, so this problem (of one cell reaching full charge before its neighbors do) is exacerbated.
While some cell technologies are fairly accommodating of problems like reverse charge and overcharging, other technologies are quite vulnerable to them Even the most accommodating technologies, however, will sooner or later lead to a xe2x80x9cbad cellxe2x80x9d in a battery, whether due to reverse charging, overcharging, or some other failure mode. When a cell goes bad, typically the battery must be removed from service and replaced by a new battery.
One approach for protecting against reverse charging is to stop using the, battery (and then to recharge it) well before any of the cells could possibly have reached full discharge. This approach leads to a rather poor ratio of weight to service life between rechargings.
One approach for protecting against overcharging is to stop charging the battery (and then to start using it) well before any of the cells could possibly have reached full charge. This approach, too, leads to a rather poor ratio of weight to service life between rechargings.
Experience shows that the life of a particular cell is not typified by perfect and identical performance for many cycles, followed by catastrophic failure as a xe2x80x9cbad cell.xe2x80x9d Instead, the life of a particular cell is typified by slight degradations in performance over its life, followed by catastrophic failure. (Of course, most cells never actually reach catastrophic failure because they are part of a battery that is removed for service because some other cell in the battery has reached catastrophic failure.) Slight degradations include small losses in charging efficiency, and small increases in the internal resistance of the cell giving rise to waste heat during discharge and charging. Such degradations would be only a small problem if they affected all cells equally, but of course experience with actual cells shows that they are not uniform across all cells.
Such slight degradations, if not provided for in the battery system design, set the stage for imbalances which will accelerate the progress toward eventual catastrophic loss of a single cell.
This accelerated progress toward failure may be described as a xe2x80x9crace to the bottom,xe2x80x9d in which the life of the battery is essentially defined by (and United by) the life of the worst cell
It would thus be extremely desirable if a system could be devised which would minimize the extent to which non-identically of cells in a battery leads to premature catastrophic failure of any one cell. It would also be extremely desirable if such a system could likewise permit use of a battery over a wide dynamic range, e.g. from nearly full charge to nearly full discharge, thus maximizing battery life between chargings, all without taking unnecessary risk of reverse charging or overcharging.
One approach to this problem may be seen in PCT publication no. 99-21241, published Apr. 29, 1999 and entitled Improved voltaic pile with charge equalizing system. The arrangement in the PCT publication, however, has a relatively high component count. It would be desirable to devise an approach having a smaller component count.
The problem of battery failure due to failure of one cell in a rechargeable battery, and the related problem of inefficient use of a battery over its dynamic range due to differences between the performance of cells in a battery, are addressed by providing one or more capacitors selectively coupled to the various cells of the battery. The selective and repetitive coupling of capacitors to the cells permits balancing of charge among the cells. This minimizes the risk that any one cell would suffer catastrophic failure due to being fully charged or discharged prior to the other cells in the battery. This also permits making use of the battery over nearly all of its dynamic range. In this way, battery life is maximized.